Hi Fos,
Been a while! Trust you are well in Perth and that FIFO agrees with you still....
Yes, switching is not the best, but if you choose a double pole double throw (DPDT) switch with high duty contacts you should be fine.
The optimum is mercury wetted, but they are pricey and might anyway be environmentally verbotten. Something rated for non-inductive 8A continuous or better is suitable. I say non-inductive because these switches usually have tungsten or beryllium contacts, which are fine for high current and anti-arcing, but not wonderful for the tiny currents of music for speakers. You don't need these qualities, in fact, since the switching will likely be done when the amp is NOT playing, at least loud, anyway.
I'm pretty sure Altronics will have something - 174 Roe St, Perth.
Hope this helps,
Hugh